1. Technical Field
This disclosure relates to an image processing method, a program and an image processing apparatus.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier or a multifunction peripheral having the respective functions thereof in a common housing, for example, an ink jet recording apparatus is known in which a liquid drop jetting head is used as a recording head. In the ink jet recording apparatus, ink as recording liquid is jetted to a paper sheet from the ink recording head so as to carry out image forming. It is noted that the above-mentioned paper sheet on which image forming is thus carried out is not actually limited to paper, but generally means one on which liquid or such may adhere to such as an OHP, or such, which may be referred to as a to-be-recorded medium, a recording medium, recording paper, or such. As to the above-mentioned image forming, a term such as recording or printing may be used as having the same meaning.
In such an image forming apparatus, as a size of the recording liquid drop, merely on the order four sizes (i.e., corresponding to four tones), i.e., no dot, a small dot, medium dot and a large dot, for example, are available. Accordingly, it is not possible to carry out halftoning only by means of the dot size of the recording liquid drop. Therefore, in general, a dither method or an error diffusion method is known as a method for halftoning with a combination of an amplitude modulation halftoning with the use of the number of levels smaller than the original, and an area coverage modulation halftoning.
The dither method (binary dither method) is a method for binarizing by determining 1 (printing or light emitting) or 0 (not printing or not light emitting) from a comparison of a pixel tone of a corresponding coordinate point with a threshold which corresponds to each value of a dither matrix. In this method, binarized data can be obtained only from the comparison of image data to the thresholds, and thus, high speed processing is achieved.
According to the error diffusion method, an error obtained when tone image data is converted into data having the number of tone levels smaller than the original (for example, binary data) is diffused to neighboring pixels, and thus, halftoning error is reduced, together with the peripheral neighboring pixels.
Each of these dither method and error diffusion method has a way in which the number of levels is more than two. For example, Japanese Laid-open Patent Application No. 2004-112089 discloses multi-level error diffusion method.